This invention relates to the manufacture of a body having a spherical surface and more particularly to a method and apparatus for milling the surface of a segment of a sphere.
A lapping machine for lapping the spherical surface of certain products such as seal rings, bearings, globe ball valves and other products having spherical surfaces is known in the prior art as being formed by a lap having a plurality of segments each having a spherical surface. These spherical surfaces may be either convex or concave. When the segments are assembled together they form the continuous surface for applying a lap or lapping compound to the surface of the product to obtain an extremely accurate and smooth finished surface. Lapping reduces the wear of parts by providing a very uniform bearing surface.
In the prior art, the common method of machining the spherical segments is by mounting a complete assembly or table of segments together on fixtures mounted on a vertical boring mill equipped with a tracer which causes a single point cutting tool to machine the surface on all the segments together. The fixtures are mounted on a table which rotates about an axis. The cutter moves about the axis of the assembled segments and then moves gradually in step-by-step fashion radially inwardly. This is a relatively slow process since only a small amount of material may be moved during each revolution and several passes are required to complete the process. Additionally, since the assembled lap table of segments may be relatively large, e.g., the spherical surface of the assembly may be in the order of approximately seven feet, the machine for mounting such assemblies is necessarily large and expensive. Accordingly, these factors together with the required assembly set-up time results in a relatively high machining cost per segment and thus the cost of the assembly is reflective thereof.